A conventional electric power steering system (EPS) assists a steering operation of a steering wheel in a vehicle by drive force of a motor as disclosed in the following patent documents.    [Patent document 1] JP 2003-182598A    [Patent document 2] JP 2009-248885A
In the EPS disclosed in patent document 1, a failure diagnosis is performed to check whether a power relay, a motor relay and a motor drive circuit are operable normally before starting a normal control operation for assisting a steering operation of a steering wheel in a vehicle after an ignition switch is turned on. In the EPS disclosed in patent document 2, a diagnosis is performed to check whether a power relay has a short-circuit failure during a normal control operation by supplying a motor with electric power from an auxiliary power source provided in a circuit, which supplies power to the motor, without through the power relay.
According to the EPS of patent document 1, however, when the failure diagnosis for the power relay or the motor relay is performed in the normal control operation, it becomes impossible to supply power to the motor and assist the steering operation of the steering wheel. For this reason, the failure diagnosis is not performed during the normal control operation. According to the EPS of patent document 2, the auxiliary power source and the power circuit are provided additionally to perform the failure diagnosis of the power relay. As a result, the EPS becomes large in physical size, the number of parts increases and a manufacturing cost increases.